Purifier comprising a photocatalytic filter

ABSTRACT

The present invention relates to a gas purifier containing a filtering media having a photocatalytic action, a system for illuminating said media with UV radiation, a time-delay means or a volatile organic compound analyzer, and a means for automatically adjusting the speed of the gas passing through the media or for adjusting the intensity of the UV illumination, said adjusting being carried out as a function of the time determined by the time-delay means or as a function of the content of a volatile organic compound analyzed by the analyzer.

The present application is the U.S. counterpart of WO 2009/019388, thetext of which is incorporated by reference, and claims the priority ofFrench Application No. 0757000, filed Aug. 8, 2007, the text of which isincorporated by reference.

The invention relates to a filtering media having a fibrous structure,the fibers of which are coated with a coating having a photocatalyticaction, for the purification of ambient air and more particularly theremoval of volatile organic compounds present in ambient air.

“Advanced Oxidation” techniques make it possible to oxidize volatileorganic compounds (VOCs). The most effective Advanced OxidationTechniques (AOTs) are those which result in the formation of hydroxylradicals OH⁻, which have a greater oxidizing power than that ofconventional oxidizing agents. This is the case with heterogeneousphotocatalysis. The fundamental principle of the phenomenon is theabsorption of a photon by a semiconducting solid, resulting in the ispromotion of an electron from the valence band to the conduction bandwith the release of a hole and thus conferring, on the solid, propertiesof oxidizing agent and of reducing agent. The majority of volatileorganic compounds and also numerous pesticides, herbicides, surfactantsand colorants are completely oxidized by this technique to give lesstoxic products.

A PCO (photocatalytic oxidation) reactor for the purification of ambientair generally comprises a prefilter for trapping dust and particles, aUV source and a PCO filter. The UV source is generally placed betweenthe prefilter and the PCO filter. The air to be purified is generallypulsed or sucked through the PCO filter using a turbine or a fan.

In order to be operational, a PCO filter has to be optimized with regardto the following points:

-   -   UV power received,    -   throughput of the purifier,    -   rate of passage of the pollutants at the medium,    -   the inertia of the media and of the PCO coating to the action of        UV radiation and hydroxyl radicals,    -   pressure drop brought about by the PCO media,    -   limitation on the creation of potentially toxic intermediate        compounds, also known under the name of by-products.

In air treatment applications, the design of the various components,fans, sheathing, engine power, is directly related to the pressure drop,which pressure drop depends on the various filtration components of thesystem, including the PCO media. This point is fundamental both from theviewpoint of the costs of the air treatment units and from its energyoperating costs. It is the present inventors who have discovered theimportance of the question of the pressure drop generated by thefiltering media.

The filters already provided for this type of application often cause anexcessively large pressure drop, so that they require the use of fanswhich are more powerful and greedier for energy. In order to overcomethis disadvantage, provision was then made to lower the density of thefilter by insertion of components, such as honeycomb, cloth with a highdegree of porosity, mosquito screen or ceramic foam, but then truepreferential channels were created and the efficiency of the filter forthe oxidation of volatile organic compounds was reduced thereby as aresult of the small amount of “effective” material in contact with theair stream.

WO 03/010106 teaches the deposition of photocatalytic coating at thesurface of silica veils or felts with a specific surface at least equalto 10 m²/g, in particular at least equal to 30 m²/g. This document doesnot suggest the notion of a low pressure drop combined with asatisfactory efficiency in the targeted application.

Mention may also be made, as documents of the prior art, of U.S. Pat.No. 4,732,879 A1. This document teaches the deposition of a porouscatalytic coating on a flexible fibrous substrate composed of glass orceramic fibers. This document suggests the use of such substrate infilter bag applications.

EP 1 132 133 teaches a photocatalytic reactor made of TiO₂ on puresintered silica. Such a sintering confers high rigidity on thestructure, which is not desired as it may be desired to fold it.Furthermore, it is clearly seen, in FIG. 3 of this document, that a highpressure drop is exerted. This is because, at 1 m/second (i.e.,approximately 3600 m³/h), the pressure drop is 200 Pa in the best ofcases. Such a product is very friable and very brittle, if it is thin.Because of its fragility, it is not possible to reduce its thickness inorder to reduce the pressure drop put forward by it.

WO 00/25919 and WO 00/76660 teaches the use of a mat of needled cutyarns as support for a photocatalytic coating. The choice of thesefibers requires the use of an organic binder and results in a mat ofhigh density (150 to 600 g/m²). It is not possible with such fibers tohave, at the same time, a low density and a low pressure drop.

WO 99/64364 (or EP 1 084 086) teaches an adhesion promoter forphotocatalytic coating. This adhesion promoter is organic.

U.S. Pat. No. 6,241,856 teaches a volatile organic compound analyzer anda pump which causes the gas to circulate but which is not under anycircumstances under the control of the analyzer. The pump thus does notserve to regulate the gas passing through the purifier according to theresult provided by the analyzer.

Mention may be made, as other documents of the state of the art, of U.S.Pat. No. 6,358,374 and WO 03/037389.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a purifier of the present invention containing two layersof media.

FIG. 2 depicts a purifier of the present invention containing one layerof media.

FIG. 3 depicts media having a “V” or “W” shape in filter modules.

FIG. 4 graphically depicts purification results corresponding to example24.

FIG. 5 graphically depicts purification results corresponding to example25.

FIG. 6 graphically depicts purification results corresponding to example26.

FIG. 7 graphically depicts purification results corresponding to example27.

FIG. 8 graphically depicts purification results corresponding to example28.

FIG. 9 graphically depicts purification results corresponding to example29.

FIG. 10 graphically depicts purification results corresponding toexample 30.

FIG. 11 graphically depicts purification results corresponding toexample 31.

FIG. 12 graphically depicts purification results corresponding toexample 33.

FIG. 13 graphically depicts purification results corresponding toexample 34.

FIG. 14 depicts the principle of illumination of the filtering media vialight guides for a reactor.

FIG. 15 depicts the principle of illumination of the filtering media viaa light guide for a reactor.

FIG. 16 depicts the principle of illumination of several filteringmedias via light guides for several reactors connected to a single UVgenerator.

The invention relates to a stable, essentially mineral, filtering mediawhich is insensitive to UV radiation and to PCO oxidation, this mediacombining a pressure drop compatible with the requirements of airtreatment systems and high effectiveness towards atmospheric pollutantsdue to its active surface and a PCO effect throughout its volume and notsolely a surface effect, as is normally encountered according to theprior art. This filtering media is preferably essentially mineral (i.e.inorganic), which means that its loss on ignition is less than 0.1% byweight and even less than 0.01% by weight, indeed even zero. Such afiltering media can in particular be obtained by the use of a feltproduced by stretch-blow molding its fibers, which makes it possible todispense with the use of binder and even any mechanical uniting action(needling, stitching).

The filtering media according to the invention is obtained afterdeposition of a coating having a photocatalytic action on a known wovenof the felt type.

The invention relates first to a filtering media having a photocatalyticaction which has a thickness of at least 2 mm, which is homogeneous andwhich is devoid of orifice apparent to the naked eye, comprising a feltof mineral fibers, the fibers of which are coated with a coatingcomprising a catalyst having a photocatalytic action, said feltexhibiting a weight per unit area of between 30 and 80 g/m², saidcoating representing 5 to 80% of the weight of said media, said mediaexhibiting a gas pressure drop of less than 150 Pa at 1 m/s in unpleatedcondition.

The invention also relates to the use of this media in variousapplications, to its shaping, in order to increase the “frontal” activesurface area as much as possible, and to its shaping, in order to reducethe problems of pressure drop as much as possible.

Moreover, the invention relates to a process and to a device for varyingthe UV intensity and the rate at the substrate at the start of apurifier and/or in the case of a pollution peak, so as to reduce theformation of intermediate reaction compounds which may be toxic.

Finally, other subject-matters of the invention are the applications ofthe PCO media and of the PCO purifying system in the treatment ofgaseous ozone effluents in domestic or industrial interior airatmospheres or alcohol and solvent vapors in industries which are majorusers of these products (composites, manufacture of fragrance, and thelike).

The media according to the invention can be employed to purify theatmosphere of premises for domestic use (dwelling) or premises of theservice sector (building containing offices). Generally, use may be madeof a felt with a weight per unit area ranging up to 300 g/m². However,the felt used in the context of the present invention, as a result ofits weight per unit area of between 30 and 80 g/m², presents a very lowpressure drop to the gas passing through it. The felt and the mediaaccording to the invention are more suitable for purification in thedomestic environment. It should be noted that, for the service orindustrial sector, substrates with a greater weight per unit area may benecessary, such as, for example, from 200 to 300 g/m². An alternativefor the service and industrial sectors is the use of several mediaaccording to the invention placed in series one behind the other. Forthese sectors, use may also be made of a single filtering mediacomprising a felt on which has been applied the catalytic coating asexplained in the present patent application, except that the feltexhibits a weight per unit area of greater than 80 g/m², for example 80to 300 g/m². It is also possible to place several of these media inseries one behind the other.

The felt to be used as substrate can exhibit a density of less than 30kg/m³. The felt to be used as substrate generally exhibits a densityranging from 0.5 to 60 kg/m³ and more generally from 1 to 30 kg/m³.

The photocatalytic coating formed according to the invention at thesurface of the fibers of the felt which is used as substrate representsfrom 5 to 80% and generally from 10 to 50% of the weight of thefiltering media.

The catalyst having a photocatalytic action generally comprises at leastone oxide from the group of the following oxides: TiO₂, ZnO and CeO₂. Itpreferably comprises titanium oxide, at least partially crystalline.

The felt is a fibrous structure having mineral fibers. These fibers canbe based on silica, such as glass (generally comprising at least 30% byweight of silica, it being possible for the glass to be of the E, C, R,S, D or AR type), washed glass (glass fiber leached chemically and thenpossibly stabilized thermally, generally comprising more than 90% byweight of silica and in standard fashion between 96% and 99% by weightof silica), of ceramic (mention may be made of fibers based on mullite,of which Unifrax and Thermal Ceramics are well known suppliers, Nextelfibers from 3M or the pure alumina fiber sold under the tradenameSaffil) or pure silica (also known as quartz and comprising at least 99%of amorphous SiO₂).

Some glass compositions suitable in the context of the present inventionare given in table 1 below:

TABLE 1 Type E glass Type C glass Type AR glass SiO₂ 53-55% 60-65% 61%Al₂O₃ 14-15% 3.5-6%   / CaO 17-23% 14%  5% MgO   1%  3% / Na₂O₃ 0.8% 10%17% B₂O₃ 0-8%  5% / Fe₂O₃ 0.3% 0.5%  0.3%  TiO₂ 0.5% / / ZrO₂ / / 10%

Use may also be made of the metal fiber (generally based on 316 or 316 Lstainless steel, the main suppliers of which are Bekaert and Ugitech).The material used is preferably glass and more preferably silica inorder to be as transparent as possible to UV illumination in use, as UVradiation then penetrates better to the core of the filtering media inorder to render it more active.

The felt is preferably nonsintered and devoid of organic matter, whichis possible in particular by the use of the stretch-blow molding processdescribed below applied to mineral material.

In order to produce a felt of mineral fibers, in particular comprisingsilica (glass or pure silica), it is possible, for example to proceed bydrawing rods of the material under consideration (such as of silica orof glass, as the case may be), with a diameter of generally less than 7mm, in a burner (in particular oxy-propane burner) in order to bringthem to a filament diameter of less than 0.5 mm. This filament can thenbe drawn again by flame stretch-blow molding in a second burner andthrown onto a surface in forward progression, such as a forwardlyprogressing belt or a rotating receiving drum. The filaments thusobtained generally have a diameter of less than 50 μm and are optimallycentered on 9 μm, for example between 7 and 15 μm. Larger filamentsgenerally cause the felt to lose flexibility. Several tens of filamentscan be drawn simultaneously by this process. The drawn material can beof quartz, silica or glass type and more generally any type ofheat-fusible mineral material, which includes alumina and mullite. Thisprocess for nonwoven (or mat) manufacture by flame stretch-blow molding,followed by throwing onto a surface in forward progression (forwardlyprogressing belt or rotating receiving drum), results in a particularlyhomogeneous structure devoid of orifices apparent to the naked eye, evenat a very low weight per unit area. This technique producessubstantially curled fibers, which means that they naturally unite withone another by intertwining to form a coherent nonwoven mat, without itbeing necessary to use a binder or to carry out a mechanical uniting,such as needling or stitching. This curling is more easily obtained byadjusting the flame which produces the drawing under turbulentconditions.

A person skilled in the art moreover clearly knows that the needling ofmineral fibers produces holes visible to the naked eye and that needledmats are not homogeneous for a weight per unit area of less than 150g/m². Conventional web-forming techniques do not make it possible toproduce homogeneous mats which hold together well below a weight perunit area of 200 g/m². Techniques for spinning through bushings resultin fibers requiring the use of organic binders in order to give cohesionto the nonwoven. In point of fact, these organic binders decompose underthe action of UV radiation, which damages the mat and is capable ofgenerating VOCs.

The pure silica fiber (at least 99% of SiO₂) is particularly preferredas it is particularly transparent to UV radiation, which allows it totransport UV radiation in all the media in the fashion of optical fibersand with a minimum of absorption.

The felt obtained by this process (flame stretch-blow molding, followedby throwing onto a surface in forward progression) is a nonwoven, theweight per unit area of which can be adjusted according to the speed ofthe receiving system (such as a rotating drum). The receiving system isadjusted so as to obtain a weight per unit area of between 30 and 80g/m². These felts exhibit a thickness ranging from 1 to 200 mm and adensity of less than 60 kg/m³. The fibers of this nonwoven have a lengthgenerally ranging from 3 cm to 100 cm. This stretch-blow molding processmakes possible the preparation of mats of pure silica or of glass (atleast 60% of SiO₂ in the case of glass). These mats are excellent andpreferred as they are flexible (as nonsintered) and devoid of anyorganic matter.

The final media exhibits a thickness which is generally less than thatof the felt used and which generally ranges from 1 to 50 mm and moregenerally between 2 and 30 mm.

It is possible, alternatively to this drawing process, to start frompreexisting fibers with a diameter of 7 to 14 μm which are cut tolengths of less than 150 mm and generally of greater than 45 mm. The cutfibers are then put into the form of a web, either by pneumatic webforming or by carding-web forming. The web thus formed is subsequentlysubjected to preneedling, followed by needling in a vicinity of 100strokes/m². By this methodology, webs with a weight per unit area of 60g/m² and 2000 g/m² can be produced. In the PCO applications to which thepresent invention relates, products with a grammage of less than 80 g/m²will be favored. These products exhibit thicknesses generally of lessthan 30 mm and generally densities of less than 70 kg/m³ and even ofless than 60 kg/m³. The density and the thickness can be adjusted by aperson skilled in the art according to the number of strokes/m² ofsubstrate practiced by the needling, which gives the felt a greater orlower density.

The media can also be prepared by the paper making route (dispersion ofthe fibers in a pulper, followed by forming by the wet paper makingroute) using, in order to retain an essentially mineral structure, abinder which is a ceramic precursor, preferably of the sol-gel type, inparticular with a precursor for example of the TEOS (tetraethylorthosilicate) or MTES (methyltriethoxysilane) type, which, aftercalcination, will be converted to ceramic. This binder can be depositedlocally pointwise or according to a predefined pattern in order tosafeguard the flexibility of the felt.

The processes which have just been mentioned produce a felt withoutholes visible to the naked eye.

The fiber preferred as backing felt for the catalyst having aphotocatalytic action is a quartz fiber (at least 99% of silica) as itwithstands well the conversion of the silica sol-gel to ceramic (between400 and 600° C.), it is very pure, devoid of alkalines and isparticularly inert with regard to the catalyst, and, furthermore, itconducts UV radiation very well without absorbing it.

The completed felt is subsequently impregnated with a solutioncomprising an organic precursor of silica (such as TEOS, MTES, indeedeven a mixture of several precursors of alkoxysilane type with thechemical formula R′_(x)Si(OR)_(4-x), in which R and R′ are organicradicals and x is an integer ranging from 0 to 3) and a dispersion of acompound having a photocatalytic action, such as TiO₂ having aphotocatalytic action or zinc oxide (ZnO), the titanium oxidenevertheless remaining the favored catalyst as a result of its highefficiency in PCO applications. The invention also relates to a processfor the manufacture of a media comprising a stage of impregnation of thefelt of inorganic fibers with a composition comprising tetraethylorthosilicate (TEOS) and at least one alkoxysilane of formulaR′_(x)Si(OR)_(4-x) in which R and R′ are organic radicals and x is aninteger ranging from 0 to 3, the amount of alkoxysilane representingfrom 10 to 40% and preferably from 15 to 25% of the weight of TEOS.

The impregnation solution can be prepared according to the instructionspresent in WO 97/10186 and WO 03/087002. By way of example, theimpregnation solution can be prepared by premixing a solution A (silicaprecursor) and a solution B (surfactant), a dispersion of titanium oxidesubsequently being added to said premix. By way of example, it can beprepared on the basis of the ingredients shown in table 2 below:

TABLE 2 Supplier and Reactant reference Weight (kg) Solution A TEOSProlabo 5 ref. 24 004.290 Absolute ethanol Prolabo 8.34 ref. 20 821.467Demineralized H₂O Prolabo 4.29 pH 1.25 (1M HCl) ref. 30 024.290 SolutionB Block polymer of BASF 3.85 propylene oxide and of PE6200 ethyleneoxide Absolute ethanol Prolabo 40.28 ref. 20 821.467 Addition of A + B61.76 A to B Catalyst C P25 (TiO₂ at 19.3% in Degussa 33.33 water)Addition of C to C + (A + B) 95.09 (A + B)

The amount of water is adjusted in order to obtain a volume of 100liters for the final solution. Two solutions A and B are thus preparedand mixed, and then a suspension of catalyst TiO₂ in water is added tothis mixture of A+B. The 33.33 kg of C are the weight of 19.3%suspension of the catalyst (not of pure catalyst).

A second example of the preparation of an impregnation solution is givenin Table 3 below:

TABLE 3 Supplier and Weight Reactant reference (kg) Solution A TEOS (d =0.93) Prolabo 9.90 (The ingredients ref. 24 004.290 are mixed untilAbsolute ethanol Prolabo 8.34 clear and then (d = 0.79) ref. 20 821.467heating is carried Demineralized H₂O Prolabo 4.29 out at 60° C. for 1 hpH 1.25 (1M HCl ref. 30 or at 50° C. for 024.290) longer) Solution BBlock polymer of BASF 3.85 (The ingredients propylene oxide and ofPE6800 are mixed until the ethylene oxide PE6800 has Absolute ethanolProlabo 40.28 dissolved) (d = 0.79) ref. 20 821.467 Addition of A to BA + B 66.67 Catalyst C TiO₂ at 19.3% in water Millenium 33.33 S5 300AAddition of C to C + (A + B) 100 (A + B)

A composition which is particularly suited to the deposition of thecoating involves, as silica precursor, a mixture of MTES and of TEOS.This is because the sol-gel obtained from this mixture is more flexibleand less subject to dust formation if compared with a 100% TEOS or 100%MTES precursor. Preferably, use is made of a mixture of 15-30% of MTESper 85-70% of TEOS.

The felt is impregnated in a full bath with the impregnation solution,the latter being sucked through the felt, which is subsequentlyexpressed (which means: pressed in order to remove the impregnationliquor) and dried.

The felt obtained is subsequently calcined in a temperature of fromambient temperature to 550° C., in particular at approximately 450° C.,which then makes it possible to convert the silica precursor to silica.Preferably, the rise in temperature up to the maximum temperature iscarried out with a moderate rate, preferably of less than 6° C. perminute. By way of example, this heat treatment can be that shown intable 4 below:

TABLE 4 Rate of temperature Stationary rise up to the Temperature phasestationary phase Ambiant 100° C.  2 h 3° C./min 150° C.  2 h 3° C./min175° C.  2 h 2° C./min 200° C. 10 min 3° C./min 300° C.  1 h 2° C./min450° C.  1 h 1° C./min

The cooling can be natural cooling in ambient air.

In the case of the use of Si and Ti precursors to produce the coatinghaving a catalytic action, the ratio of the Si and Ti precursors ispreferably varied in order for the Si/Ti molar ratio in the coatinghaving a catalytic action to be between 0.25 and 1.35 and morepreferably between 0.5 and 1.3.

The filtering media having a photocatalytic action according to theinvention is thus obtained. This media can be sucked via a suction tablein order to remove the particles (micronic and submicronic) of coatingexhibiting low adhesion. This makes it possible to avoid significantdust formation by the PCO media and generation of particles during thefirst startups of the PCO purifier.

It can be fitted to cartridge and photocatalytic reactor systems.

The weight of catalyst (such as TiO₂) is generally less than or equal to40% by weight and, if possible, less than 30% by weight, optimally ofapproximately 15% by weight, with regard to the final product (media)obtained. Preferably, the weight of catalyst is greater than or equal to1% by weight and more preferably greater than or equal to 5% by weight,with respect to the final media.

In the case of a TiO₂ catalyst, the latter preferably comprises as muchas possible of anatase.

In some types of application (bactericidal application, destruction ofozone and sulfur compounds of H₂S or DMDS (dimethyl disulfide of formulaCH₃—S—S—CH₃) type), the composition can be doped with at least onecompound, such as MnO, Mn₂O₃, dicyanoanthracene (DCA) or a compoundcomprising at least one of the elements from the group of the elementsV, Cr, Mn, Mo, In, Sn, Fe, Ce, Co, Cu, Nd, Zn, W, Nb, Ta, Bi, Ni, Ru andAg, said compound being at a concentration of less than 0.5% by weightof the weight of catalyst, in order to increase the efficiency of themedia.

The felts prepared as indicated above exhibit the following properties:

-   -   they are essential of mineral nature;    -   they exhibit a pressure drop of less than 150 Pa at 1 m/s and        more generally of less than 50 Pa at 1 m/s and even of less than        20 Pa at 1 m/s of gas;    -   they are homogeneous and thus without a preferential pathway for        the gas passing through it;    -   they do not exhibit holes visible to the naked eye.

The preparation of the photocatalytic coating can sometimes presentproblems of adhesion to the fiber, in particular when the latter issubjected to mechanical stresses, even weak mechanical stresses, forexample during handling. The detachment of the coating is reflected bythe formation of an undesired dust. This detaching is also known as“dust formation”.

In order to reduce this formation of dust, it is possible to reduce thethickness of the coating. It is also possible to attach the coatingusing a polymer. However, the latter has to be able to resist oxidationunder the joint effect of UV radiation and the PCO effect of thecatalyst, the powers received very often being between 2 and 40 mW/cm²of UV-A, UV-B or UV-C.

It has been discovered that the polymers comprising fluorine, such aspolytetrafluoroethylene (PTFE), or a fluorosilane (such as thefluorosilanes sold under the references F8820, F8810 and F8263 byDegussa), and to a lesser extent a polysiloxane polymer (such as asilicone), have sufficient stability towards UV radiation and towardsthe PCO reaction in this type of application. The polymer can bedeposited on the fibers of the media in the form of a dispersion in aliquid, in particular using an aqueous dispersion. Use is preferablymade of an aqueous dispersion of the polymer devoid of surfactant orcomprising as little as possible of surfactant. This is because, withsome types of surfactant used for these dispersions, for example fordispersing PTFE, a strong odor may appear in the application at thebeginning of the use of the filtering media, as a result of thedecomposition of the surfactant under the PCO action. Appropriatepolymer dispersions are, for example, the PTFE dispersions sold underthe references Teflon 30 B, Teflon 304 A, Teflon B and Teflon-3823, soldby DuPont de Nemours. Mention may also be made of silicone polymers fromRhodia Silicone, such as 83% Rhodorsil Resin 20 B or Rhodorsil Resin6405, or siloxane polymers from Wacker, such as Silres H62C. PTFE is apreferred polymer.

The polymer dispersion is applied to the media after the ceramizationheat treatment which has resulted in the formation of the coating havinga photocatalytic action. Generally, from 0.1 to 5% by weight of polymer,with respect to the weight of the final media, is deposited on themedia. The deposition of the polymer can be carried out either byspraying on one or both faces with the suspension or byimmersion-dipping in the suspension, followed by expressing. Theseimpregnations can generally be carried out at ambient temperature, inparticular at a temperature of between 10 and 40° C. For the case wherethe dispersion of the polymer comprises a surfactant, preferably a heattreatment is carried out generally between 45° C. and 250° C.(especially between 150 and 250° C. in the case of a fluoropolymer (notof the polysiloxane type), for example a few minutes at 230° C., andespecially between 100 and 180° C. in the case of a fluorinatedpolysiloxane) or a UV treatment is carried out (in this case, at a highUV intensity, in particular between 15 and 100 mW/cm²) after theapplication of the polymer dispersion and before the true use in orderas best as possible to remove the surfactants used to disperse thepolymer in the dispersion, in particular fluoropolymers in aqueousdispersion.

Fortuitously, it has furthermore been discovered that the PCOsubstrates, the coating of which comprises a hydrophobic polymer, inparticular a fluoropolymer or a polymer of the polysiloxane type, canfloat on the surface of the water. Such media are highly advantageous inpurifying the atmosphere irradiating from a settling tank, from awater-treatment plant, from a (factory) lagoon, and the like. This isbecause the media can be cut into surface fragments ranging from a fewmm² to a few cm² run onto the surface of the water to be treated. As aresult of its floating nature, the media is easily distributed at thesurface of the water (without major additional cost, such as floatingcontainers fitted to floats). The media then adsorbs the pollutantsemanating from the polluted water and oxidizes them under the action ofsolar UV radiation. This principle is highly economic in verysubstantially reducing the emanations, foul-smelling and sometimesdangerous in terms of chemical compounds, from this type ofinstallation. Thus, the invention also relates to a process for thepurification of the air above water comprising impurities which generatevolatile organic compounds in the air above said water, by virtue of theplacing, at the surface of the water, of a filtering media having aphotocatalytic action which is self floating (according to theinvention) by means of a suitable coating which allows it to float. Thiscoating preferably comprises a hydrophobic polymer applied to the fibersof said media.

The media according to the present invention comprises a surface whichis active and homogeneous (without apparent preferential pathway for theair) over the entire surface of the media with a very low pressure drop,in particular as a result of its very low grammage (or weight per unitarea) and its low associated bulk density. Furthermore, as the media isvery thick, it makes it possible to have a photocatalytic oxidationeffectiveness (PCO effectiveness) throughout its thickness.

A simple way of characterizing photocatalytic activity of a media is totest in traversing mode the percentage of decomposition of a specificpollutant, such as methanol, in a laboratory reactor. To this end, areactor developed by the team of Professor Pichat of l'Ecole Centrale deLyon is commonly used in universities and laboratories. This reactor isgenerally composed of a body made of stainless steel inside which isplaced a disk of media, for example with a diameter of 47 mm. UVillumination from an HPK 125 W lamp is carried out through a silica slitin the top part of the reactor. The illuminating power is regulated byadjusting the lamp/media distance. A power of 5 mW of UV-A per cm² ofmedia, measured at 365 nm and measured on the media, is generally used.A continuous flow of filtered air comprising 300 ppm of pollutant (inparticular methanol) is introduced at a rate of 350 ml/min upstream ofthe reactor. The concentration of pollutant is then measured downstreamof the reactor, after PCO treatment, generally by chromatography. It isimportant to make sure that the pollutant has been converted to minerals(conversion to CO₂ and H₂O) by checking the chemical balance in ordernot to have solely phenomena of pure adsorption, as may be found withsystems of the active charcoal type. This test, applied to methanol, isreferred to subsequently as the “methanol test”.

The filtering media according to the invention, in unpleated condition,causes a pressure drop of less than 150 Pa at 1 m/s of gas and evengenerally of less than 50 Pa at 1 m/s of gas and even of less than 20 Paat 1 m/s of gas, which is remarkably low, while providing excellentpurification.

In the case of a high concentration of pollutant in the atmosphere to betreated (for example during pollution peaks or at the beginning of thedecontamination treatment by the filtering media according to theinvention, which can easily be detected by sensors of volatile organiccompounds), the formation of intermediates, such as formaldehyde,acetaldehyde or acetone, becomes more pronounced as the flow rate and/orthe UV power increases. As some of these derivatives are particularlytoxic, it is then recommended (if these particular conditions occur), inorder to overcome their formation, briefly to:

-   -   greatly reduce the UV intensity and/or    -   greatly reduce the flow rate of the purifier, that is to say the        rate of passage of the gas to be treated through the filtering        media.

By combining these two actions, a photocatalytic purifier makes possibleefficient purification of the air without increasing the levels ofharmful intermediates during the first minutes of the start of thedevice or of flow of the pollutant peak. Subsequently, the UV intensityand the flow rate of the device can be returned to their nominal valuein order to provide a maximum PCO effect.

This reduced operating capacity is to be followed at the beginning ofoperation, when the room or atmosphere is highly charged with VOC. Thisis because, in this case, the VOC in high concentration at the startgenerates other VOCs by decomposition on contact with the filteringmedia, said other VOCs being themselves in relatively highconcentration, and the combination of a high concentration of VOC and ofa high gas rate then perhaps does not allow the filtering media to“convert to minerals” all the VOCs in the purifier. In other words,there would be so much VOC in the purifier that a high proportion ofthese VOCs would risk passing through the purifier. This risk decreasesthe smaller the starting concentration of VOC as then the filteringmedia surface area becomes sufficient to convert to minerals all theVOCs, intermediate or not. It is thus indeed the same chemicaldecomposition reactions of the VOCs in the purifier which take placebut, in the case of a high concentration of VOC in the atmosphere to bepurified, there is a risk that the process of conversion to minerals(sequence of chemical reactions) will not be able to go to completionbecause of saturation of the purifier.

As example of a sequence of chemical decomposition reactions in thepurifier, a process for the decomposition of methanol is:methanol→formaldehyde→formic acid→CO₂. As example of a sequence ofchemical reactions, a process for the decomposition of ethanol is:ethanol→acetaldehyde→acetic acid→formaldehyde→formic acid+CO₂→2 CO₂.Depending on the characteristics of the photocatalytic purifier (type ofmedia, air rate, level of UV illumination) and the concentration of VOCat the inlet (pollutant) of the purifier, the intermediate reactionproducts may or may not be completely converted to minerals during thepassage over the photocatalytic media of the initial molecule ofstarting pollutant.

It is possible, in order to further increase the efficiency of thephotocatalytic reactor system according to the invention, to vary theincrease in the surface area of active media and the reduction in thepressure drop. If the installation of several filtering media in seriesmakes it possible to increase the active surface area, the pressure dropis, however, correspondingly increased thereby. In order to achieve thisobjective, it is possible to combine the increase in surface area ofactive photocatalytic media with an increase in the frontal surfacearea, the pressure drop decreasing as the frontal surface area incontact with the air flow increases. To this end, the media according tothe invention can be positioned in a filtration cartridge so as topresent a longer surface to the gas to be treated. Thus, instead ofbeing positioned in a simple linear thickness transversely with respectto the direction of the gas, it is possible to confer on it a shapecomprising at least one angle, such as a V shape, W shape, and the like(until a true “accordion” is formed) or to give it a pleated structure.The media can also be placed in articulated cartridges which follow thepreceding designs, allowing the cartridge to be placed in a reactor witha minimum of lost space. This point is particularly advantageous indomestic air conditioning systems. This is because, very often, thesuction conduits are very quickly bent at an angle behind the dustfiltration components. Insofar as the PCO purifier systems are placed inthe dust filtration region, the bulkiness of the PCO reactor has to belimited. Such an articulated cartridge system makes it possible to avoidthe bulkiness of an angled system for its installation while benefitingfrom the angled shape once in a working position, which makes itpossible to reduce the pressure drop of the system. The cartridge canthus be inserted straight (without an angle) into the slit which makesit possible to introduce it into the PCO reactor and the angled shape isautomatically assumed inside the reactor at the time of the insertion.FIGS. 3 a and 3 b show PCO filters inside which the PCO media has a V orW shape. Thus, the invention also relates to a filter cartridgecomprising at least one angle and comprising a PCO media, said anglebeing, if appropriate, articulated.

The PCO purifier according to the invention is intended in particularfor domestic air purification applications. One of the majorapplications is the reduction of ozone in a domestic environment. Byvirtue of the invention, an ozone reduction efficiency of 90% can beachieved.

The PCO purifier according to the invention is also used for thepurification of air in the service, commercial or industrial sector. Inthis type of application, it is recommended to use media according tothe invention in series in order to generate a higher weight per unitarea in comparison with that which is suitable for domesticsurroundings. For example, in the context of use in an industrialcatering environment equipped with a system for cleaning grease(starting with ozone), a 50 g/m² media generally does not make itpossible to lower ozone levels, sufficiently high at a level of 300 ppb,in one pass at 1 m/s. It is then necessary to use medias according tothe invention in series (that is to say, one after the other, ifappropriate in contact), generally between 2 and 10 medias, moreparticularly from 3 to 6 medias, which makes it possible in one pass tooxidize, for example, 150 ppb of the 300 ppb of ozone present in theinlet gas, the UV power received per filter being 50 mW/cm² of UV-Cillumination. This type of process can be envisaged in particular inindustrial kitchen implementers in which ozone is generated in order toremove the grease deposited in the extraction hoods of the kitchens.This results in a strong smell and high concentration of ozone which canbe destroyed by the PCO system according to the invention.

The PCO system according to the invention can also be used in industrialapplications, such as warehouses or refrigerators for the storage ofeasily damaged plant products (such as fruit, vegetables, flowers). Inthis context, it is important to reduce the concentration of ethylene inthe warehouse in order to slow down the ripening of the fruit or thewilting of the flowers. The invention consequently relates to the use ofthe media or of the purifier or of the device according to the inventionto purify the air of a warehouse or of a refrigerator containing aplant, in particular a fruit or vegetable or flower.

The PCO system according to the invention is also very efficient withregard to the decomposition of alcohols (methanol, ethanol, propanol)and solvents, for example used in the resin or composite industry or inthe manufacture of fragrances. In this type of application, it isnecessary to operate under EXAT regulated conditions (the expressionEXAT originating from “EXplosive ATmospheres”) in order to avoid anyrisk of explosion. One of the problems is the nature of the PCOfiltering media, which is actually essentially mineral in order to avoidany risk of ignition. In particular, preferably, for EXAT applications,postimpregnation polymer is not applied to the media in order to limitdust formation. The media of the present invention is then composed ofan mineral substrate (felt) with a mineral coating doped with titaniumoxide and corresponds fully to the specifications for PCO applicationsin EXAT surroundings.

The PCO system according to the invention comprises a source of lightnecessary for the catalytic activation of the titanium oxide coating.This source can be a UV-A, UV-B or UV-C mercury vapor lamp or a xenonlamp.

Furthermore, the invention also relates to devices for the specificillumination of a PCO media suitable for the PCO media according to theinvention or any other PCO media. These devices are advantageous inparticular from the viewpoint of the energy saving, of reduction inmaintenance costs or of EXAT conformity.

The device for illuminating the fibrous media can be produced with aUV-A, UV-B or UV-C LED with an illuminating power of at least one 1mW/cm². Such a system makes it possible to combine a minimum energyconsumption with significant efficiency. Moreover, this type ofilluminating system allows very specific designs with optimization ofthe illumination of the media. Thus, the invention also relates to a gaspurifier comprising a filtering media having a photocatalytic action(according to the invention or not according to the invention) and asystem for illuminating said media with UV, said illuminating systemcomprising an LED, preferably UV, generating an intensity received bythe media at least equal to 1 mW/cm² of media.

The device for illuminating the fibrous media can be produced with alight guide, for example an optical fiber: this system makes it possibleto move the source away from the illuminating region and thus itrelatively easily creates an EXAT region in the PCO reactor. Thus, theinvention also relates to a gas purifier comprising a filtering mediahaving a photocatalytic action (according to the invention or notaccording to the invention) and a system for illuminating said mediawith UV radiation, said illuminating system comprising at least onelight guide (such as an optical fiber) in order to bring the light tosaid media. In industrial applications, it is not uncommon to find EXATregions (outlets, uptakes). Currently, conventional UV lamps are notEXAT approved because of the lack of strength of their tube. A systemfor lighting with a light guide makes it possible to take the lamp outof the EXAT region and thus renders the system in accordance with therequirements of EXAT regions. The following systems can be envisaged:

-   -   starting from a UV source, it is possible to introduce several        strands of light guide (for example of optical fiber type) into        the reactor. These strands act as guide for introducing the        light energy into the reactor. For example, it is possible to        envisage a strand every cm² of substrate, so as to distribute        the UV energy as homogeneously as possible in the reactor. The        principle of such a system is represented in FIG. 14.    -   starting from a source, it is possible to introduce a single        strand of light guide (such as optical fiber) into the reactor.        This strand then illuminates a reflector or a mirror provided        with a suitable curvature and the UV light is reflected by this        mirror or this reflector as homogeneously as possible in the        reactor. The principle of this system is represented in FIG. 14.        The use of any other scattering system may also make it possible        to achieve this objective.

The device for illuminating the fibrous media can also be centralizedwith regard to several PCO reactors. This makes it possible to have asingle region for the generation of UV radiation relayed, via a systemfor illuminating with a light guide (for example of optical fiber type)different PCO media. This system provides a not insignificant energysaving by avoiding the need to have multiple sources and by reducing theconsumption of ballasts and the inevitable losses due to multisourcesystems. The principle of this system is represented in FIG. 16. Thus,the invention also relates to a device for purifying the air comprisingseveral air purifiers each comprising a filtering media having aphotocatalytic action (according to the invention or not according tothe invention) and comprising a single source of lighting of the mediasof the purifiers.

Thus, the invention also relates to a gas purifier comprising afiltering media having a photocatalytic action (according to theinvention or not according to the invention) and a system forilluminating said media with UV radiation, said illuminating systemcomprising light guides (for example of optical fiber type) and/or acold light.

The device for illuminating the fibrous media can be produced with aflat lamp. This system makes possible an extremely homogeneousillumination of the entire surface of the PCO media and therebyincreases the oxidation yield and thus the efficiency of the system.Thus, the invention also relates to a gas purifier comprising afiltering media having a photocatalytic action (according to theinvention or not according to the invention) and a system forilluminating said media with UV radiation, said illuminating systemcomprising a flat lamp.

When the PCO system is operated in an atmosphere polluted with VOCs, theconcentration of these VOCs can be fairly high at the beginning ofoperation. This high concentration of VOCs can be reflected by theundesired formation, subsequent to the operation of the PCO systemaccording to the invention, of intermediate compounds (formaldehyde,acetaldehyde, acetone) which are also toxic. This is why it isrecommended, in the case of a VOC concentration which is believed to behigh, to start the operation of the PCO system according to theinvention in an attenuated mode, either by reducing the UV power or byreducing the flow rate of gas or both. After a certain time, when theVOC concentration is lower, it is possible to increase the operatingpower. A weakened UV illumination (for the beginning of operation) is,for example, less than 8 mW/cm² and even less than 7.5 mW/cm². Aweakened gas flow rate is, for example, less than 60% and even 50% ofthe nominal gas flow rate. The concentrations of some VOCs measured instandard fashion in residential rooms are given in table 5 below. Thesevalues are the results of several hundred measurements. The right-handcolumn “critical concentration” of table 5 shows, by way of indication,the concentrations above which is recommended to reduce the operatingcapacity of the PCO purifier according to the invention by reducing theflow rate of gas passing through it and/or by reducing the UV intensityilluminating the PCO media. The invention thus relates to a process ofthe purification of gas using a purifier comprising a filtering mediahaving a photocatalytic action (according to the invention or notaccording to the invention) and a system for illuminating said mediawith UV radiation, so that, when the concentration of a compound in thegas is greater than a value V1, the operating capacity of the purifieris lower than its capacity when the concentration of the compound in thegas is less than a value V2, V2 being less than or equal to V1. In thecase of formaldehyde (very common impurity), it is recommended to reducethe operating capacity of the purifier when the concentration offormaldehyde is greater than 30 μg/m³. The operating capacity can beincreased when the concentration of formaldehyde is less than 30 μg/m³.More generally, the purifier can be operated with a reduced operatingcapacity when the concentration of formaldehyde is greater than a valueV1 between 0.3 and 80 μg/m³ and then the operating capacity can beincreased when the concentration of formaldehyde is less than a value V2between 0.3 and 80 μg/m³, V2 being less than or equal to V1.

TABLE 5 Normal values (residential rooms) Critical Median MinimumMaximum concentration μg/m³ μg/m³ μg/m³ μg/m³ Formaldehyde 24.0 2.0 74.830.0 Hexaldehyde 17.0 0.7 138.0 22.0 Toluene 15.6 3.6 145.2 20.0Acetaldehyde 12.0 1.4 78.0 15.0 Limonene 8.9 1.5 71.2 11.0Isobutyraldehyde/ 8.8 0.7 24.0 11.0 butyraldehyde Undecane 6.9 1.1 146.28.0 α-Pinene 5.9 0.7 262.1 7.0 Decane 5.9 0.7 105.5 7.0 Valeraldehyde5.0 0.7 27.0 6.0 (m + p)-Xylenes 4.7 1.6 76.7 6.0 1,2,4- 2.4 0.7 55.44.0 Trimethylbenzene Isovaleraldehyde 2.1 2.1 3.0 4.0 Ethylbenzene 2.00.7 24.5 4.0 o-Xylene 1.8 0.7 24.6 4.0 Benzene 1.8 0.7 14.1 4.01-Methoxy-2-propanol 1.7 0.7 32.1 4.0 Tetrachloroethylene 1.4 0.7 73.64.0 Butyl acetate 1.4 0.7 40.9 4.0 1,4-Dichlorobenzene 1.4 0.7 293.2 4.02-Ethyl-1-hexanol 1.0 0.7 12.1 3.0 2-Butoxyethanol 0.7 0.7 14.0 2.01,1,1-Trichloroethane 0.7 0.7 6.1 2.0 Trichloroethylene 0.7 0.7 41.8 2.0Styrene 0.7 0.7 5.3 2.0 2-Ethoxyethanol 0.7 0.7 7.6 2.0 2-Ethoxyethylacetate 0.7 0.7 2.2 2.0 Benzaldehyde 0.7 0.7 2.0 2.0

In order to be able to detect if the gas to be purified (generally theair) exceeds the values for which it is recommended to reduce the powerof the purifier, the purifier according to the invention isadvantageously provided with a volatile organic compound analyzer. Thepurifier can operate completely automatically according to the contentsof volatile organic compound transmitted by the analyzer: high capacitywhen the content is lower than a certain value, low capacity when thecontent is greater than a certain value.

Thus, the invention also relates to a gas (generally the air) purifiercomprising a filtering media having a photocatalytic action (accordingto the invention or not according to the invention) comprising a meansfor varying the flow rate of gas passing through it or (which meansand/or) for varying the intensity of the UV illumination. The purifiercan comprise a volatile organic compound analyzer and a means forautomatically adjusting the rate of the gas passing through it or foradjusting the intensity of the UV illumination according to the contentof volatile organic compound analyzed by the analyzer. The purifier cananalyze the incoming gas or the exiting gas but generally analyzes theincoming gas. An example which may be given of a suitable deviceoperating according to this principle is the operating mode whichfollows, according to which, above the content C1 of a VOC, the deviceoperates in reduced mode. Above a content C1 of a VOC, the analyzergives a signal 1 which enters a regulator, which is programmed toconvert this signal 1 to an outlet instruction 1 in accordance with amathematical formula, said instruction then operating a variable speeddrive which controls the speed (reduced) of the drive engine of the fanof the purifier, said fan driving a flow rate D1 (moderate) of gasthrough the purifier.

After a certain operating time at this reduced capacity and when theconcentration of said VOC in the gas falls below C2 (C2 being lower thanC1), then the analyzer gives a signal 2 to the regulator, which isprogrammed to convert the signal 2 to an outlet instruction 2 inaccordance with a mathematical formula, said instruction then operatingthe variable speed drive which controls the speed (high) of the drivemotor of the fan of the purifier, said fan driving a flow rate D2 (high)of gas through the purifier.

The gas speed and/or the light intensity can also be very simplyadjusted as a function of the time. For example, when a purifier isstarted in a room comprising pollutants, it is recommended to operatewith a low gas speed and/or low UV illumination for example over 2 h,the time to have sufficiently purified the room, and then to change tonominal capacity. Such a system prevents by-products from being formed,for example when the purifier is started. The purifier can thus comprisea time-delay means (that is to say a means which measures or determinesthe time or which triggers a device at the end of a certain time) whichmakes it possible to control the moderate or higher capacity of thepurifier. The purifier can thus comprise a time-delay means and a meansfor automatically adjusting the speed of the gas passing through itand/or for adjusting the intensity of the UV illumination as a functionof the time determined by the time-delay means. Recourse can be had tothis time-based system, during the detection of a pollution peak by asystem which is or is not independent of the purifier, such as, forexample, information communicated on the radio, to place the system inreduced-capacity mode (low gas rate and/or low UV illumination) for apredefined time, the greater purification capacity subsequently beingautomatically engaged from the end of the predefined time. In such adevice, by way of example, after starting the purifier, the controlsequence, including a time-delay relay, gives an instruction 1 forreduced operation for a period of time 1 to a variable speed drive,which gives the speed instruction 1 to the drive motor of the turbine ofthe fan in order to entrain a flow rate D1 (reduced) of the gas throughthe purifier. After a predefined period of time at this reducedcapacity, the control sequence, including the time-delay relay, gives aninstruction 2 for operating at a greater capacity to the variable speeddrive, which gives the speed instruction 2 to the drive motor of theturbine of the fan in order to entrain a flow rate D2 (greater) of gasthrough the purifier.

FIG. 1 represents very diagrammatically the structure of a PCO purifieraccording to the invention comprising two layers 2 of PCO media. The gasflow is represented by thick arrows, the left-hand arrow representingthe incoming gas and the right-hand arrow representing the exiting gas.UV lamps 3 are placed between the two layers 2 of PCO media. A fan 5provides air circulation. All these components are placed in a stainlesssteel chamber 1. The distance 4 between the UV lamp and the medias canbe 20 mm. Such a purifier might furthermore comprise a particleprefilter at the start of the air arrival, that is to say placed to theleft of the first PCO media.

FIG. 2 represents the structure of a PCO purifier according to theinvention comprising just one layer of PCO media 2. The air flow isrepresented by thick arrows. A UV lamp 3 illuminates the media 2. A fan5 provides air circulation. All these components are placed in a chambermade of stainless steel 1. The distance 4 between the UV lamp and themedia can be 20 mm. Such a purifier might furthermore comprise aparticle prefilter as soon as air arrives, that is to say placed to theleft of the first PCO media.

FIG. 3 show PCO filter modules inside which the PCO media has the shapeof a V or W. The module comprises a generally metallic (stainless steel,galvanized steel or aluminum) housing 6 comprising a particle prefilter7, a UV lamp 8 and an articulated or nonarticulated cartridge 9 having aphotocatalytic media. The cartridge goes into the unit via an opening10. Depending on the place available for the opening of the housing, itis decided if the cartridge should or should not be articulated. Forexample, an articulated cartridge can go straight into the modulethrough the opening 10 (FIG. 3 c) and then folded into a V by virtue ofthe articulation 11 (see FIG. 3 a). The cartridge can comprise threearticulations 12 in order to assume the shape of a W, as in FIG. 3 b.

FIGS. 4 to 13 give the results for the purification, by virtue of amedia according to the invention, of air polluted by various molecules.FIGS. 4 to 11 correspond to the results of examples 24 to 31 and FIGS.12 and 13 correspond to the results of examples 33 and 34.

FIG. 14 represents the principle of the illumination of the filteringmedia via several light guides (for example optical fibers) conveyed tothe PCO reactor 15. In FIG. 14 a), it is seen that, starting from thesource of UV light 13, several strands of light guide 14 convey thelight to the reactor 15 for the purpose of illuminating the PCO media.In FIG. 14 b), the distribution in the arrival of eight light guides 14on the section of the reactor 15, so as to distribute the UV energy ashomogeneously as possible in the reactor 15, is seen.

FIG. 15 represents the principle of the illumination of the filteringmedia via a single light guide (for example optical fiber) conveyed tothe PCO reactor 21. Starting from the UV source 16, a single strand 17of light guide conveys the UV light to the reactor 21. In the caserepresented, the light guide passes through the media 18 to theillumination point 19 at the end of the light guide. This strand thenilluminates a mirror 20 provided with an appropriate curvature and theUV light is reflected (arrows) by this mirror as homogeneously aspossible in the reactor towards the media 18.

FIG. 16 represents the principle of the illumination of several PCOmedias present in several PCO reactors 22 via light guides (for exampleoptical fibers) 23 connected to a single UV generator 24. Theilluminating device is thus centralized with regard to several PCOreactors. This makes it possible to have a single region of generationof UV radiation relayed by an illuminating system with a light guide tovarious PCO medias. This system avoids the need to have multiplesources.

EXAMPLES 1 to 13 Evaluation of Dust Formation

Felts are produced in the following way. Molten silica rods with adiameter of 4.4 mm are drawn in an oxy-propane burner in order to bringthem to a filament diameter of 0.2 mm. This filament is then drawn againby flame stretch-blow molding in a second burner in order to obtain amean diameter of 9 μm and thrown onto a receiving belt or drum. Thespeed of the drum is adjusted so as to obtain the weight per unit areaof the felts which appear in the table below (2^(nd) table). The feltobtained is then impregnated with the preparation obtained by theformulation shown in table 6 below:

TABLE 6 Supplier and Reactant reference Weight (kg) Solution A TEOSProlabo X (The ingredients ref. 24 004.290 are mixed until MTES DegussaY clear and then Absolute ethanol Prolabo 8.34 heating is carried ref.20 821.467 out at 60° C. for Demineralized Prolabo 4.29 1 h or at 50° C.H₂O at ref 30 024.290 for longer) pH 1.25 (1M HCl) Solution B Blockpolymer of BASF 3.85 (The ingredients propylene PE6800 are mixed withoxide and of heating at 50° C. ethylene oxide until the PE6800 Absoluteethanol Prolabo 40.28  has dissolved) ref. 20 821.467 Addition of A + BA to B Catalyst C 19.3% TiO₂ Millenium Z in water S5 300A Addition of Cto To be adusted (A + B) with water for a total of 100 liters

Various tests were carried out with different amounts of TEOS, MTES andcatalyst TiO₂ as reported in table 7 below.

TABLE 7 PCO Dust TEOS MTES TiO₂ efficiency formation Example (X) (Y) (Z)Si/Ti Other parameters (ppm) (mg/m²) No. 1 — Reference: no impregnation0 0.2 (comparative) No. 2 — Example 10, impregnation 35 0.6 solution ofwhich has been diluted 5 fold in water No. 3 9.9 10.6 1.79 120 0.7 No. 49.9 15.15 1.25 180 0.8 No. 5 9.9 22.72 0.83 200 2.9 No. 6 9.9 22.72 0.83Sucked 90 s on a suction table 200 1.3 with a suction rate of 1 m/s No.7 9.9 22.72 0.97 180 1.3 No. 8 3.96 5.94 22.72 0.92 160 1 No. 9 7.622.27 22.72 0.87 200 0.75 No. 10 9.9 33.33 0.57 200 0.95 No. 11 9.9 22.720.83 With postimpregnation with 150 0.95 silane Dynasilan 8820 fromDegussa at a level of 0.4% No. 12 9.9 22.72 0.83 Spraying of Rhodorsilover the 170 0.8 2 faces of the media at a level of 0.3% No. 13 9.922.72 0.83 Spraying of PTFE over the 2 230 0.1 faces of the media at alevel of 0.3% No. 13a 9.9 76.03 0.25 Millenium S5 300 A 150 7 No. 13b9.9 63.36 0.3 Millenium S5 300 A 160 6 No. 13c 7.67 2.27 97.66 0.2Millenium S5 300 A 140 8 No. 13d 7.62 2.27 14.07 1.4 Millenium S5 300 A130 0.7

The impregnated felt is subsequently subjected to a heat treatment underthe conditions of table 4 already seen above. The final media obtainedexhibits a weight per unit area after impregnation of approximately 120g/m² and an apparent thickness of 20 mm and its content of titaniumoxide is of the order of 20% by weight (except for case No. 1: nocatalyst, and No. 2: less than 4% of catalyst).

Measurements for evaluating the tendency towards dust formation of thecoating are subsequently carried out starting from 100×100 mm² samplesof PCO media. The samples are placed in a sieve of Fritsh/Labogerdebautype lasting 30 minutes with an amplitude of 4 (value specific to themachine). At the end of the 30 minutes, the residue is weighed and theloss in weight is related to 1 m² of substrate. The values are meanedover 5 samples. The results are reported in comparative fashion in table7. The results are expressed as “dust formation”, that is to say theamount of dust formed per unit of surface area of media (in mg/m²) and aPCO efficiency on the basis of the “methanol test” already describedabove. This efficiency is expressed as amount of methanol oxidized inppm. In particular, test No. 13 is excellent as it combines a very goodPCO activity with very low dust formation.

EXAMPLES 14 to 23 Oxidation of Methanol

Use is made of a reactor composed of a body made of stainless steelinside which is placed a disk of media with a diameter of 47 mm. HPK 125W UV illumination is produced in the top part of the reactor through asilica slit. The illuminating power is regulated by adjusting thelamp/media distance. A power of 5 mW/cm² at 365 nm (of media) of UVmeasured at the media is used. Upstream of the reactor, a continual flowof filtered air comprising 300 ppm of methanol is introduced at the rateof 350 ml/min. The concentration of methanol is measured by gaschromatography downstream of the reactor, after PCO treatment.Conversion of the pollutant to minerals (conversion to CO₂ and H₂O) isascertained by confirming the chemical balance in order not to havesolely phenomena of pure adsorption as may be encountered with systemsof the active charcoal type. The more specific operating conditions forproducing the media and also the efficiency observed with regard todifferent medias in the context of the “methanol test” already explainedabove are given in table 8. In Millenium S5 300 A, the TiO₂ was 100%anatase. This is not necessarily the case for any catalyst based onTiO₂. For example, in the catalyst P25 from Degussa, the TiO₂ comprisesapproximately ⅓ of rutile per ⅔ anatase.

TABLE 8 PCO Example Felt TEOS MTES TiO₂ TiO₂ Other efficiency No.grammage (X) (Y) (Z) origin Si/Ti parameters (ppm) 14 200 g/m²  9.933.33 Millenium 0.57 240 S5 300 A 15 80 g/m² 9.9 33.33 Millenium 0.57200 S5 300 A 16 80 g/m² 9.9 33.33 Millenium 0.57 Suction of the 200 S5300 A catalyst preparation 90 s on a suction table with a suction rateof 1 m/s 17 65 g/m² 9.9 33.33 Millenium 0.57 170 S5 300 A 19 80 g/m² 9.933.33 Millenium 0.57 Spraying of PTFE 230 S5 300 A over the 2 faces ofthe media at a level of 0.3% 20 80 g/m² 7.62 2.27 33.33 Millenium 0.59200 S5 300 A 21 65 g/m² 9.9 33.33 Millenium 0.66 180 S5 300 A 22 (comp.)Media sold by Toshiba with a density of 0.7 g/cm³ Ceramic foam 170 in athickness of 10 mm PCO 23 (comp) Paper PCO + active charcoal, Ahlstromwith the Polyester felts 110 reference 1054 PCO + active charcoal

EXAMPLES 24 to 31 Oxidation of Organic Molecules at Various Flow Ratesand Intensities of Illumination

Molten silica rods with a diameter of 4.4 mm are drawn in an oxy-propaneburner in order to bring them to a filament diameter of 0.2 mm. Thisfilament is subsequently drawn again by flame stretch-blow molding in asecond burner in order to obtain a mean diameter of 9 μm and thrown ontoa receiving drum. The speed of the drum is adjusted so as to obtain aweight per unit area of the felt of 80 g/m². The felt is thenimpregnated in accordance with the formulation of Table 9 below:

TABLE 9 Supplier and Weight Reactant reference (kg) Solution A TEOSProlabo 7.00 (The ingredients ref. 24 004.290 are mixed until MTESDegussa 2.9 clear and then Absolute ethanol Prolabo 8.34 heating iscarried ref. 20 821.467 out at 60° C. for 1 h) Demineralized H₂O atProlabo 4.29 pH 1.25 ref 30 024.290 (1M HCl) Solution B Block polymer ofBASF 3.85 (The ingredients propylene oxide and of PE6800 are mixed withethylene oxide heating at 50° C. Absolute ethanol Prolabo 40.28 untilthe PE6800 ref. 20 821.467 has dissolved) Addition of A to B A + B 66.67Catalyst C TiO₂ (19.3% in water) Millenium 33.33 S5 300A Addition of Cto C + (A + B) 100 (A + B)

The impregnated felt is subsequently subjected to a heat treatment underthe conditions of table 4 already seen above.

The filtering media obtained exhibits an apparent thickness of 20 mm.The felt exhibits a total weight after impregnation of 120 g/m², thelevel of titanium oxide being 20% of the total weight of the media.

The efficiency of the photocatalytic media was measured by placing it ina PCO purifier with a flow rate of 130 m³/h, the frontal velocity being1 m/s and the illuminating power received being 15 mW of UV-C per cm² ofmedia (these values were measured with a Bioblock VLX-3W radiometer witha 254 nm probe). The chamber is made of stainless steel and measures 1m³. The purifier is made of stainless steel. It is equipped with 3Philips 36 W TUV lamps placed at 20 mm from the PCO media or medias. Themedia surface area is 270×420 mm². The PCO purifier comprises one or twomedia, as represented in FIGS. 1 and 2. The fan is placed behind the PCOfilter or filters.

A pollutant mixture is introduced into the chamber at the rate of 1.95l/min using a permeameter. This mixture is composed of benzene, toluene,o-xylene, decane, limonene and formaldehyde.

Fresh filtered air is introduced at the rate of 21 l/min and an outletsystem pumps 23 l/min of the atmosphere, in order to simulate the degreeof replacement of fresh air existing in any building. In the case of ahigh VOC concentration and in order to reduce the formation of areaction intermediate which may be toxic, such as formaldehyde,acetaldehyde or acetone, it is important to operate with levels of UVillumination which are not excessively high and moderate flow rates, inorder to increase the reaction time within the substrate and to makepossible more complete oxidation of the organic compounds (whichincludes these possible undesired intermediate compounds) within thesubstrate.

The meanings of the various abbreviations are as follows:

1F: 1 PCO media 2F: 2 PCO medias on either side of the UV lamp Dmax:Maximum flow rate (130 m³/h) D1/2: Maximum flow rate divided by 2 UVmax: Maximum UV illumination UV/2: Maximum UV illumination divided by 2UV min: UV illumination received 2 mW/cm².

The results are given by FIGS. 4 to 11. The time during which thepurifier operates is indicated in the figures by the double-headed arrow“purifier in operation”.

The oxidation of the various compounds benzene, toluene, decane, xyleneand limonene by virtue of the PCO filter according to the invention isvery marked from the various curves. Correspondingly, it is apparentthat the formation of intermediates, such as formaldehyde, acetaldehydeor acetone, becomes more pronounced as the flow rate and UV powerincrease.

EXAMPLE 32 Ozone

Molten silica rods with a diameter of 4.4 mm are drawn in an oxy-propaneburner in order to bring them to a filament diameter of 0.2 mm. Thisfilament is then drawn again by flame stretch-blow molding in a secondburner in order to obtain a mean diameter of 9 μm and thrown onto areceiving drum. The speed of the drum is regulated so as to obtain aweight per unit area of the felt of 65 g/m². The product is thenimpregnated with the preparation obtained according to the formulationof table 10 below:

TABLE 10 Supplier and Weight Reactant reference (kg) Solution A TEOSProlabo 5.00 (The ingredients ref. 24 004.290 are mixed until MTESDegussa 4.9 clear and then Absolute ethanol Prolabo 8.34 heating iscarried ref. 20 821.467 out at 60° C. for 1 h) Demineralized H₂O atProlabo 4.29 pH 1.25 ref 30 024.290 (1M HCl) Solution B Block polymer ofBASF 3.85 (The ingredients propylene oxide and of PE6800 are mixed withethylene oxide heating at 50° C. Absolute ethanol Prolabo 40.28 untilthe PE6800 ref. 20 821.467 has dissolved) Addition of A to B A + B 66.67Catalyst C 19.3% TiO₂ in water Millenium 33.33 S5 300A Addition of C toC + (A + B) 100 (A + B)

The impregnated felt is subsequently subjected to a heat treatment underthe conditions of table 4 already seen above.

The filtering media obtained exhibits an apparent thickness of 20 mm.The felt exhibits a total weight after impregnation of 100 g/m², thelevel of the titanium oxide being 20% by weight of the total weight ofthe media.

The tests were carried out in an experimental house. The purifier wasthat described in examples 24 to 31, equipped with 2 medias.

The test conditions were as follows:

-   -   office of 30 m³    -   degree of replacement of air (DRA) between 0.6 and 1 volume/h    -   felts of fibers of molten silica of the Quartzel trademark        (registered trademark of Saint-Gobain Quartz SAS) and with a        weight per unit area of 65 g/m² (media weight 100 g/m²)    -   nominal flow rate, purifier 130 m³/h    -   UV illumination received 15 mW/cm²

The measurements were carried out one week before installing thepurifier, one week during the operation of the purifier and one weekafter halting the purifier. The results are expressed as ratio ofinternal ozone concentration to external ozone concentration. This isbecause, under real conditions, as a degree of replacement of air alwaysexists, fresh air laden with pollutant enters the room and stale airladen with pollutant exits from the room. In order to compareefficiencies, it is therefore useful to be able to operate in relationto internal air pollutant concentration/external air pollutantconcentration. Without a PCO purifier, this ratio is 0.14. With the PCOpurifier, this ratio is 0.01.

EXAMPLES 33 and 34

Molten silica rods with a diameter of 5.5 mm are drawn in an oxy-propaneburner in order to bring them to a filament diameter of 0.2 mm. Thisfilament is then drawn again by flame stretch-blow molding in a secondburner in order to obtain a mean diameter of 9 μm and thrown onto areceiving drum. The speed of the drum is regulated so as to obtain aweight per unit area of the felt of 50 g/m². The felt is thenimpregnated with the preparation prepared according to the formulationof table 11 below:

TABLE 11 Supplier and Weight Reactant reference (kg) Solution A MTESProlabo 9.90 (The ingredients ref. 24 004.290 are mixed until Absoluteethanol Prolabo 8.34 clear and then ref. 20 821.467 heating is carriedDemineralized H₂O at Prolabo 4.29 out at 60° C. for 1 h) pH 1.25 ref 30024.290 (1M HCl) Solution B Block polymer of BASF 3.85 (The ingredientspropylene oxide and of PE6800 are mixed with ethylene oxide heating at50° C. Absolute ethanol Prolabo 40.28 until the PE6800 ref. 20 821.467has dissolved) Addition of A to B A + B 66.67 Catalyst C 19.3% TiO₂ inwater Millenium 33.33 S5 300A Addition of C to C + (A + B) 100 (A + B)

The impregnated felt is subsequently subjected to a heat treatment underthe conditions of table 4 already seen above. The filtering mediaobtained exhibits an apparent thickness of 20 mm. The felt exhibits atotal weight after impregnation of 85 g/m², the level of titanium oxidebeing 20% of the total weight of the media.

The media is then placed in a purifier identical to that described forexamples 24 to 31 (a single PCO media) with the following parameters:

-   -   nominal flow rate in the purifier: 130 m³/h;    -   UV illumination received: 15 mW/cm².

The purifier to be tested is placed in a Plexiglas chamber with a volumeof one m³.

Prior to the test, the chamber is purged with ultrapure and humidifiedair in order to remove the presence of pollutant before the introductionof the mixture of model molecules. A liquid mixture of the variouspollutants is introduced through a septum via a syringe into a glassweighing boat. Two mixtures were tested, one comprising propionaldehyde,heptane, acetone, toluene, acetaldehyde, ethylene, styrene and o-xylene(example 33) and the other comprising toluene, heptane, butyraldehyde,acetone and methoxyethanol (example 34). After evaporation, theconcentration of the various compounds is of the order of a ppmv. TheCO₂ is monitored with a gas microchromatograph equipped with a thermalconductivity detector (μGC-TCD) and the other pollutants are analyzedwith a gas chromatograph equipped with a photoionization detector (PID).The PID makes it possible to analyze ionizable volatile organiccompounds (VOCs) in the ppbv range. The possible presence ofdecomposition by-products in the gas phase is detected by adsorption onan adsorbent cartridge (flow rate 100 ml/min, time 20 minutes), followedby thermal desorption analysis coupled to a gas chromatograph anddetected by mass spectrometry. The results are given in FIGS. 12 and 13.The PCO system according to the invention is found to be notablyeffective in removing the various solvents as soon as the UVillumination is triggered (the moment of illumination is indicated bythe arrow “UV in operation”). In the case of example 34 (FIG. 13),acetaldehyde is momentarily formed beginning with the UV illuminationand is then itself oxidized. The methoxyethanol peak between 40 and 70min corresponds to a second injection of this product into the chamber.The affinity of methoxyethanol for the media is such that it isimmediately adsorbed.

1. A gas purifier comprising a filtering media having a photocatalyticaction, a system for illuminating said media with UV radiation, atime-delay means or a volatile organic compound analyzer, and a meansfor automatically adjusting the speed of the gas passing through themedia or for adjusting the intensity of the UV illumination, saidadjusting being carried out as a function of the time determined by thetime-delay means or as a function of the content of a volatile organiccompound analyzed by the analyzer.
 2. The purifier as claimed in claim1, wherein it comprises a volatile organic compound analyzer and theanalyzer analyzes the incoming gas.
 3. The purifier as claimed in claim1, said illuminating system comprising an LED generating an intensityreceived by the media of at least equal to 1 mW/cm² of media.
 4. Thepurifier as claimed in claim 1, said illuminating system comprising atleast one light guide for conveying the light to said media.
 5. Thepurifier as claimed claim 1, comprising a filter cartridge comprising atleast one angle, said cartridge comprising the filtering media having aphotocatalytic action.
 6. The purifier as claimed in claim 5, whereinthe angle is articulated.
 7. A device for purifying air comprisingseveral purifiers as claimed in claim 1, wherein said device comprises asingle source for lighting the medias of the purifiers.
 8. The device asclaimed in claim 1, wherein the lighting is transmitted to the purifiersvia optical fiber light guides.
 9. A process for the purification of agas comprising passing the gas through the purifier of claim 1 .
 10. Aprocess for purifying gas comprising passing the gas through a purifiercomprising a filtering media having a photocatalytic action and a systemfor illuminating said media with UV radiation, wherein, when theconcentration of a compound in the gas is greater than a value V1, theoperating capacity of the purifier is lower than its capacity when theconcentration of the compound in the gas is less than a value V2, V2being less than or equal to V1.
 11. The process as claimed in claim 10,wherein the compound is formaldehyde and V1 and V2 are between 0.3 and80 μg/m³.
 12. The process as claimed in claim 10, wherein the purifiercomprises a filtering media having a photocatalytic action, a system forilluminating said media with UV radiation, a time-delay means or avolatile organic compound analyzer, and a means for automaticallyadjusting the speed of the gas passing through the media or foradjusting the intensity of the UV illumination, said adjusting beingcarried out as a function of the time determined by the time-delay meansor as a function of the content of a volatile organic compound analyzedby the analyzer.
 13. A process for purifying gas comprising controllingthe speed of the gas passing through a purifier comprising a filteringmedia having a photocatalytic action, a system for illuminating saidmedia with UV radiation and a time-delay means controlling a moderate orgreater capacity of the purifier, wherein the time-delay means controlsthe speed of the gas passing through the purifier.
 14. A method forpurifying the air of a workshop for the manufacture of fragrance or of adistillery comprising passing the air through a gas purifier as claimedin claim
 1. 15. A method for purifying the air of a warehouse or of arefrigerator containing a fruit or a vegetable or a flower comprisingpassing the air through a gas purifier as claimed in claim
 1. 16. Aprocess for purifying gas passing through a purifier comprising afiltering media having a photocatalytic action, a lamp for illuminatingsaid media with UV radiation and a time-delay means controlling amoderate or greater capacity of the purifier comprising controlling thespeed of the gas passing through the filtering media and/or controllingthe intensity of the UV illumination delivered by the lamp, wherein thetime-delay means controls the speed of the gas passing through thepurifier and/or the intensity of the UV illumination delivered by thelamp.
 17. The process as claimed in claim 16, wherein the speed of thegas passing through the filtering media is controlled by the time-delaymeans.
 18. The process as claimed in claim 16, wherein the intensity ofthe UV illumination delivered by the lamp is controlled by thetime-delay means.